1. Field of the Invention
The present invention relates to a piezoelectric element having an electromechanical energy conversion function, in which piezoelectric layers and electrode layers are laminated, and more particularly, to a piezoelectric element suitable for a vibration wave motor and a piezoelectric actuator, and to a piezoelectric actuator and a vibration wave motor which include the piezoelectric element.
2. Description of the Related Art
A piezoelectric material is a typical material having an electromechanical energy conversion function for converting electrical energy into mechanical energy.
Owing to its characteristics, a piezoelectric element made of a piezoelectric material is used in various piezoelectric actuators such as a vibration wave motor.
In particular, in recent years, not only a single plate piezoelectric element but also laminated piezoelectric elements are used, such as an element in which multiple piezoelectric layers and electrode layers are laminated alternately to be multilayered and baked integrally, and an element in which piezoelectric elements baked into single plates are laminated and bonded.
This is because the laminated structure can obtain a larger displacement or a larger force with a low voltage than a single plate piezoelectric element. In particular, the integrally baked laminated piezoelectric element is suitable for reduction in size and thickness.
Conventionally, as to a vibration wave motor as an oscillatory wave driving device, there are many proposals about the laminated piezoelectric element constituting a part of a vibration body of a rod-like vibration wave motor. This laminated piezoelectric element includes multiple piezoelectric layers made of a piezoelectric material and electrode layers (referred to also as internal electrodes) made of an electrode material and disposed on the surfaces of the piezoelectric layers.
For instance, U.S. Pat. No. 5,770,916 discloses a laminated piezoelectric element as illustrated in FIG. 9 as a laminated piezoelectric element used for a vibration body of a rod-like vibration wave motor.
Multiple electrode layers 43 are disposed on the surfaces of multiple piezoelectric layers 42 except the uppermost layer constituting a laminated piezoelectric element 40.
The electrode layer 43 is divided into four regions (in FIG. 9, eight types of A+, A−, B+, B−, AG+, AG−, BG+, and BG− are illustrated).
Further, connection electrodes 43a (part filled with black color in FIG. 9) are formed to be connected to the electrode layers 43 and extend to outer edge portions of the piezoelectric layers 42.
The connection electrodes 43a are connected to external electrodes 44 disposed on the outer peripheral portion of the laminated piezoelectric element 40 for electrical connection among layers.
Multiple surface electrodes 45 are disposed on the surface of the uppermost piezoelectric layer 42 constituting the laminated piezoelectric element 40 and are each formed corresponding to a position of the connection electrode 43a so as to be connected to the external electrode 44.
Polarization is performed by applying DC voltages via the surface electrodes 45 to A+, A−, B+, and B− of the electrode layers 43 so that A+ and B+ become positive while A− and B− become negative with respect to AG+, AG−, BG+, and BG− that are the ground. Thus, the polarities for driving the vibration wave motor can be obtained.
Further, FIG. 10 is a cross sectional view illustrating an example in which the laminated piezoelectric element 40 of FIG. 9 is incorporated in a vibration body 51 of a rod-like vibration wave motor 50.
In FIG. 10, the laminated piezoelectric element is interposed between hollow metal members 53 and 54 constituting the vibration body 51 together with a flexible circuit board 52 and is fastened by a bolt 55.
On one side of the vibration body 51 in the axial direction, there is disposed a rotor 58 that is pressed by a spring 56 and a spring support 57 to contact with a distal end portion of the metal member 54, and hence a rotation output can be obtained from a rotating gear 59.
As a method of driving the rod-like vibration wave motor 50, AC voltages having different temporal phases by approximately π/2 are applied to Phase A constituted of A+, A−, AG+, and AG− and to Phase B constituted of B+, B−, BG+, and BG− of the laminated piezoelectric element 40 incorporated in the vibration body 51.
As a result, two bending vibrations orthogonal to the axial direction are excited instead of rotating wobbling motion. Then, using the distal end portion of the metal member 54 constituting the vibration body 51 as a frictional surface, the rotor 58 that is pressed to contact with the metal member 54 rotates by the friction contact.
As described above in the conventional example, the polarization is performed in which DC voltages are applied to A+, A−, B+, and B− so that A+ and B+ become positive while A− and B− become negative with respect to AG+, AG−, BG+, and BG− that are the ground. In this case, there is a known problem that a non-electrode portion 46 made of the piezoelectric layer between two neighboring electrode portions to which DC voltages having opposite polarities (positive and negative) are applied as illustrated in FIG. 9 causes a potential difference and is polarized in the polarization.
Herein, the region in which the piezoelectric layer 42 and the electrode layer 43 are overlaid (the region in which the electrode layer is formed on the piezoelectric layer) is referred to as the electrode portion, while the region having only the piezoelectric layer 42 without the electrode layer (the region of the piezoelectric layer in which the electrode layer is not formed) is referred to as the non-electrode portion.
When the non-electrode portion is polarized, the Young's modulus as a mechanical property of the non-electrode portion is changed by the polarization, and there occurs a difference in Young's modulus between the polarized non-electrode portion 46 and an unpolarized non-electrode portion 47. As a result, the piezoelectric layer having the non-electrode portion 46 and the non-electrode portion 47 has locally different Young's modulus values. Further, the non-electrode portion 46 and the non-electrode portion 47 have a positional relationship in which they pass a center axis and are orthogonal to each other with respect to the piezoelectric layer. For instance, supposing the non-electrode portion 46 is the X axis, the non-electrode portion 47 becomes the Y axis, and the X axis and the Y axis are orthogonal to each other. In this way, if there are regions having different Young's modulus values in the X axis direction and in the Y axis direction, the difference in Young's modulus between the non-electrode portion 46 and the non-electrode portion 47 causes anisotropy (uneven stiffness) of the mechanical property (flexural stiffness) in the vibration body 51 incorporating the laminated piezoelectric element 40, resulting in uneven vibration amplitude or phase.
This influence may cause deterioration of controllability and rotational position accuracy, and cause rotational unevenness, and further hinder even friction in the rod-like vibration wave motor of the conventional example.
Therefore, in Japanese Patent Application Laid-Open No. 2003-111450, it is devised to increase a width of the non-electrode portion (distance between the electrode portions).
There is another problem that the laminated piezoelectric element is expensive. The piezoelectric material is usually baked at a relatively high temperature of approximately 1,200° C. in the case of a single plate. Then, silver paste containing glass frit is applied after the baking, and baked at a temperature of 600 to 850° C. to obtain the piezoelectric element. In this way, the baking condition is different between the piezoelectric material and the electrode material in accordance with the characteristics of the material.
On the other hand, in the laminated piezoelectric element, the piezoelectric material and the electrode are baked together, and hence it is necessary to consider the baking condition for a metal material of the electrode. As a result, as the electrode material, it is necessary to use precious metal (such as silver-palladium alloy or rarely platinum), which has a high melting point so as not to be melted at the baking temperature and which is not oxidized or hardly oxidized by baking.
Conventionally, the laminated piezoelectric element is baked using an electrode material containing silver at 70 wt % and palladium at 30 wt %, for example, at a lowered baking temperature of 1,150° C. in view of a composition of the piezoelectric material or the use of fine powder grains.
Palladium and platinum as precious metal are particularly expensive, and hence have a high cost ratio in the total cost of the electrode material, which is responsible for high cost.
Therefore, development of piezoelectric materials that can be baked at low temperature has been carried out actively, and there are many proposals for lowering the baking temperature by adding copper or other element so that low temperature baking can be performed (see U.S. Pat. No. 5,798,052).
As described above, in the vibration wave motor using the laminated piezoelectric element for the vibration body, in order to suppress the rotational unevenness or the like, it is required to reduce as much as possible the anisotropy of mechanical property due to the difference in Young's modulus generated by polarization of the non-electrode portion between two electrode portions having different polarities.
Therefore, the increase in width of the non-electrode portion as described in Japanese Patent Application Laid-Open No. 2003-111450 does not always lead to a satisfactory output of the piezoelectric element.
In addition, there is a proposal for enabling the low temperature baking of the laminated piezoelectric element as described in U.S. Pat. No. 5,798,052 and the like. However, it is desired to lower the baking temperature as much as possible while maintaining piezoelectric characteristics, so as to obtain a laminated piezoelectric element that can be manufactured at a lower baking temperature and can provide good piezoelectric characteristics.